Originally the phenomenon of gravitational lensing (GL), namely the bending of light [1]},
was the most significant demonstration of the validity of General Relativity (GR) [2]}. It has since become a fruitful and primary tool for studying some
of the most important aspects of cosmology and astrophysics, such as the distribution of dark matter in galaxy clusters [3]}. The phenomenon has been
studied in both weak field and strong field regimes [1]}. For strong gravitational fields an infinite number of images (called relativistic images)
on each side of the optical axis of a Schwarzschild black hole have been found [5]}, [6]}, [7]}. A calculation of time delay between the outermost two relativistic images has been useful in obtaining the mass of the black hole with high precision. Furthermore, a given mass and angular separation between relativistic images can be used to calculate the distance to the black hole [8]}, [9]}.
